A ferromagnetic material means a material that is spontaneously magnetized even though a strong magnetic field is not applied from the outside. A giant magnetic resistance effect that an electric resistance is changed depending on relative magnetization directions of two magnetic layers occurs in a spin valve structure having a non-magnetic material inserted between two ferromagnetic bodies (a first magnetic material/a non-magnetic material/a second magnetic material). This occurs because electric resistances experienced by up-spin and down-spin are different from each other in the spin valve structure. The giant magnetic resistance effect is widely used as a core technique of a sensor for reading data stored in a hard disk.
While the giant magnetic resistance effect describes a phenomenon that relative magnetization directions of two magnetic layers control the flow of a current, it is also possible to control a magnetization direction of a magnetic layer using an applied current according to the law of action and reaction, which is Newton's third law. A current is applied to the spin valve structure so that a current spin-polarized by the first magnetic material (a fixed magnetic layer) passes through the second magnetic material (a free magnetic layer) to transfer its spin angular momentum. This is called spin-transfer-torque.
International Business Machines Corporation (IBM) suggested a device having a free magnetic layer of which a magnetization is reversed or continuously rotated using the spin-transfer-torque. Thereafter, the device was experimentally identified. In particular, a magnetic memory device using the spin-transfer-torque is spotlighted as a new memory device replaced with a dynamics random access memory (DRAM).
A basic magnetic memory device has the spin valve structure, as described above. In other words, a conventional magnetic memory device 100 has a structure of a lower electrode/a first magnetic material 101 (the fixed magnetic layer)/a non-magnetic material 102/a second magnetic material 103 (the free magnetic layer) of which a magnetization direction is changable by a current/an upper electrode, as shown in the following FIG. 1. The magnetization reversal of the second magnetic material is induced by a current or magnetic field applied from the outside, and a high resistance and a low resistance are shown by the giant magnetic resistance effect described above. These are applied as data of “0” or “1” written in the magnetic memory device.
If an external magnetic field is used in order to control the magnetization of a free layer, a half-selected cell problem occurs with the reduction of a size of a device to limit high integration of the device. On the other hand, if the spin-transfer-torque occurring by applying a voltage to a device is used, the magnetization reversal of a selected cell is easy irrelevantly to a size of a device. According to a physical mechanism of the spin-transfer-torque described above, a magnitude of the spin-transfer-torque occurring in the free magnetic layer is proportional to the amount of an applied current density (or a voltage), and a critical current density for the magnetization reversal of the free magnetic layer exits. If all of the fixed layer and the free layer are composed of a material having perpendicular anisotropy, the critical current density Jc is expressed by the following equation 1.
                              J          c                =                                            2              ⁢              e                        ℏ                    ⁢                                    α              ⁢                                                          ⁢                              M                S                            ⁢              d                        η                    ⁢                      (                          H                              K                ,                eff                                      )                                              [                  Equation          ⁢                                          ⁢          1                ]            
In the equation 1, “α” denotes the Gilbert damping constant, “h” (=1.05×10−34 J·s) denotes a value obtained by dividing the Planck constant by 2π, “e” (=1.6×10−19 C) denotes the quantity of electrical charge of the electron, “η” denotes a spin polarization efficiency constant determined by a material and a structure of a device, “Ms” denotes a saturation magnetization of the free magnetic layer, “d” denotes a thickness of the free magnetic layer, and “HK,eff” denotes an effective anisotropy magnetic field in a perpendicular direction of a layer (HK,eff=HK⊥−4πMS).
If a size of a cell is reduced for high integration of the device, the magnetization direction written by thermal energy at a room temperature is randomly changed. This is a limitation of super-paramagnetism and causes a problem that a written magnetic data is undesirably erased. A mean maintaining time τ of the magnetization direction overcoming the thermal energy is expressed by the following equation 2.
                    τ        =                                            τ              0                        ⁢                          exp              ⁡                              (                                                                            K                      eff                                        ⁢                    V                                                                              k                      B                                        ⁢                    T                                                  )                                              =                                    τ              0                        ⁢                          exp              ⁡                              (                                                                            H                                              K                        ,                        eff                                                              ⁢                                          M                      S                                        ⁢                    V                                                        2                    ⁢                                          k                      B                                        ⁢                    T                                                  )                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
In the equation 2, “τ0” denotes an inverse number of an attempt frequency and is about 1 ns, “Keff” denotes an effective magnetic anisotropy energy density of the free magnetic layer (=HK,effMs/2), “V” denotes a volume of the device, “KB” denotes the Boltzman's constant (=1.381×10−16 erg/K, and “T” denotes the Kelvin temperature.
Here, “KeffV/kBT” is defined as a thermal stability factor Δ of the magnetic memory device. Generally, a condition of Δ>50 should be satisfied in order that the magnetic memory device maintains its non-volatile characteristic. If the volume V of the free layer is reduced with the reduction of the size of the cell, the Keff should be increased to satisfy the condition of Δ>50. As a result, the Jc increases according to the equation 1.
Thus, because the Δ and the Jc of the magnetic memory device are proportional to the Keff, a sufficient high Δ and a sufficient low Jc should be satisfied for the commercialization of the device. In addition, because the amount of a current provided in a complementary metal-oxide-semiconductor (CMOS) transistor device is limited, a low critical current density for the magnetization reversal of the free magnetic layer is required. Moreover, the reduction of the critical current density is a necessary factor in order to reduce a power consumption required for driving the device.
In other words, the magnetization reversal critical current density of the free layer should be reduced in order to reduce the size of the memory device and in order to realize high integration of the memory device. Also, the magnetization reversal critical current density of the free layer is reduced so that a power used for writing should be reduced.
As described above, because the critical current density of the magnetic memory device is proportional to the effective magnetic anisotropy magnetic field HK,eff, the effective magnetic anisotropy magnetic field HK,eff should be effectively reduced in order to reduce the critical current density of the device. A method of applying a high frequency modulation magnetic field was suggested as the above method. The high frequency modulation magnetic field was applied simultaneously with a magnetic field generated from a writing head of a hard disk drive, thereby reducing a magnitude of a writing magnetic field. This uses a principle that a frequency of an applied AC magnetic field is closer to a resonance frequency of a magnetization of a writing medium to generate the magnetization reversal with a magnetic field lower than an original HK,eff. A method of reducing the critical current density by applying the same principle to a current driving magnetic memory device was experimentally verified. However, the principle and the structure surely require an additional device for inducing the modulation magnetic field, and it was confirmed that a reduction effect of a driving power was less effective in an entire device.
Additionally, the magnetization direction of the second magnetic material is varied by the stray field generated from the first magnetic material in the conventional art shown in the following FIG. 1. In more detail, if the magnetization direction of the first magnetic material is a +z-axis being a thickness direction of a layer, a direction of the stray field also becomes the +z-axis. Under this condition, a Δ in the event that the magnetization direction of the second magnetic material is a −z-axis is smaller than a Δ in the event that the magnetization direction of the second magnetic material is the +z-axis, due to the influence of the stray field. Since the magnetic memory device has all of the directions being +z and −z directions of the second magnetic material, thermal stability of the magnetization is determined by a smaller Δ of the two events. Thus, characteristics of the device are deteriorated by the stray field generated from the first magnetic material.
Additionally, if the saturation magnetization value of a magnetic layer is 650 emu/cm3 or more, the influence of a corresponding magnetic layer on a neighboring magnetic layer increases to be likely to cause problems on characteristics of the device.